Radiographic imaging such as x-ray imaging has been used for years in medical applications and for non-destructive testing.
Normally, an x-ray imaging system includes an x-ray source and an x-ray detector array consisting of multiple detectors comprising one or many detector elements (independent means of measuring x-ray intensity/fluence). The x-ray source emits x-rays, which pass through a subject or object to be imaged and are then registered by the detector array. Since some materials absorb a larger fraction of the x-rays than others, an image is formed of the subject or object.
An example of a commonly used x-ray imaging system is an x-ray computed tomography, CT, system, which may include an x-ray tube that produces a fan- or cone beam of x-rays and an opposing array of x-ray detectors measuring the fraction of x-rays that are transmitted through a patient or object. The x-ray tube and detector array are mounted in a gantry that rotates around the imaged object. An illustration of a fan beam CT geometry is shown in FIG. 3. For a given rotational position, each detector element measures the transmitted x-rays for a certain projection line. Such a measurement is called a projection measurement. The collection of projection measurements for many projection lines is called a sinogram. The sinogram data is utilized through image reconstruction to obtain an image of the interior of the imaged object. Certain types of image reconstruction, such as iterative reconstruction or basis material decomposition, require the formation of a so-called forward model, which describes the imaging system in detail.
The dimensions and segmentation of the detector array affect the imaging capabilities of the CT apparatus. A plurality of detector elements in the direction of the rotational axis of the gantry, i.e. the z-direction of FIG. 3 enables multi-slice image acquisition. A plurality of detector elements in the angular direction (ξ in FIG. 3) enables measurement of multiple projections in the same plane simultaneously and this is applied in fan/cone-beam CT. Most conventional detectors are so called flat-panel detectors, meaning that they have detector elements in the slice (z) and angular (ξ) directions.
X-ray detectors made from low-Z materials need to have a substantial thickness in the direction of the x-ray beam in order to have sufficient detection efficiency to be used in CT. This can be solved by, for example, using an “edge-on” geometry, as in U.S. Pat. No. 8,183,535, in which the detector array is built up of a multitude of detectors, which comprise thin wafers of a low-atomic number material, oriented with the edge towards the impinging x-rays. It is common that each detector has a plurality of detector elements on a 2D grid on the wafer. Each individual wafer is, for example, oriented such that it has detector elements in the slice direction (z) and in the direction of the x-rays, as schematically illustrated in FIG. 3. The edge-on geometry for semiconductor detectors is also suggested in U.S. Pat. No. 4,937,453, U.S. Pat. No. 5,434,417, US 2004/0251419 and WO 2010/093314. Wafer detectors that are oriented with a slight angle with respect to the direction of the x-rays are normally also included in the term “edge-on”.
For edge-on detectors to function as designed, it is generally important that the detectors are oriented the way that they are designed for with respect to the direction of the impinging x-rays, or at least that the detectors orientation is known and taken into consideration. Due to uncertainties in the detector mounting and in the position of the focal spot of the x-ray tube, it is not certain that each detector is aligned as desired with respect to the direction of the x-rays. If the detectors are misaligned with respect to the direction of the x-rays and left un-corrected, it can lead to, for example: lower detection efficiency; lower spatial resolution; un-accurate forward models in the image reconstruction; and further, impaired image quality. If the orientation of the detectors with respect to the direction of the x-rays can be estimated, corrections can be made either before or after the image acquisition.
US 2014/0211925, U.S. Pat. No. 8,622,615 and US 2014/0153694 relate to geometric calibration for flat-panel detectors using a calibration phantom or device. However, for conventional flat-panel detectors, the direction of the x-rays does not have a major impact on the performance.
U.S. Pat. No. 5,131,021, U.S. Pat. No. 8,262,288, U.S. Pat. No. 6,370,218, U.S. Pat. No. 5,469,429, U.S. Pat. No. 5,131,021 relate to calibration and/or adjustment of the position of the focal spot of the x-ray tube.
However, nowhere in the prior art are there any described methods for determining parameters related to the orientation of edge-on detectors for CT.
WO 2010/093314 merely mentions the possibility to measure and correct for a mechanical misalignment of a semiconductor detector element that is segmented in the direction of the x-rays based on a model of the expected ratio of detected x-rays in the top and bottom segments.